cylc validate: multiple inheritance broken

With something like this:

```
# my-suite
[scheduling]
[[dependencies]]
graph = """foo"""
[runtime]
[[FOO]]
[[BAR]]
[[foo]]
inherit = FOO, BAR
```

`cylc validate --debug my-suite` gives::

```
Traceback (most recent call last):
  File "/opt/cylc/bin/cylc-validate", line 60, in <module>
    verbose=options.verbose )
  File "/opt/cylc/lib/cylc/config.py", line 277, in __init__
    coerce_runtime_values( self['runtime'][ns] )
  File "/opt/cylc/lib/cylc/config.py", line 90, in coerce_runtime_values
    rdict[item] = str2list( rdict[item] )
  File "/opt/cylc/lib/cylc/config.py", line 67, in str2list
    return re.split( '[, ]+', st )
  File "/usr/lib64/python2.6/re.py", line 167, in split
    return _compile(pattern, 0).split(string, maxsplit)
TypeError: expected string or buffer
```

This change should fix it.